Structural evolution of iron components and their action behavior on lignite combustion

Spontaneous combustion of lignite is closely related to the inherent minerals it contains, and the iron component has a remarkable influence on the combustion property of lignite. It is very important to study the influence of iron component on the combustion reaction property of lignite to reveal autoignition mechanism of lignite and reduce autoignition of lignite. In this research, FeCl₃ and Fe₂O₃ were doped into demineralised lignite (SL⁺) by impregnation to research the effects of iron salts and iron oxides on the combustion properties of lignite. Based on the above, the effects of post-treatment method of the FeCl₃-doped coal samples, iron-salt hydrolysis products and heat-treated temperatures on the combustion property of lignite were researched, and the microstructures of the coal samples were characterised and analysed using Fourier transform infrared spectroscopy (FTIR), Scanning electron microscope-energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results demonstrate that doping with FeCl₃ increases the combustion performance of lignite, thereby reducing the ignition temperature of lignite by approximately 112 °C. In contrast, doping with Fe₂O₃ has a weaker combustion-promoting effect. XRD and XPS characterisation indicates that iron species in the coal samples doped with iron salts are highly dispersed and exhibit the FeOOH structure, whereas iron species in the coal samples doped with Fe₂O₃ exhibit the crystal form of α-Fe₂O₃. Doping of lignite with FeCl₃ and its hydrolysis product β-FeOOH reduces the ignition temperature of the coal samples. Iron species in the FeCl₃-doped coal samples after heat treatment at 300–500 °C increase the combustion property of the coal samples, whereas iron species after heat treatment at 600–900 °C have a much weaker or non-existent promoting effect on the combustion performance of the coal samples. The characterisation show a change in iron species in the coal samples with the rise in the heat treatment temperature. This change progresses from highly dispersed β-FeOOH below 300 °C to Fe₃O₄ above 400 °C. Fe₃O₄ is gradually reduced, with part of it further reduced to elementary iron at the same time as grain growth. It is believed that the gradual agglomeration of Fe₃O₄ and the appearance of elementary iron are the main reasons for the weakening or disappearance of the promoting effect on coal combustion.